The object of the present invention, which results from the collaboration of the Service National des Champs Intenses (national department for the study of strong fields: Director, M. AUBERT) is a method for making a gradient coil as well as a gradient coil obtained by this method. This gradient coil is designed to create a magnetic field gradient in a volume covered by a nuclear magnetic resonance device for the formation of optical images. The invention has special application in the medical field where optical image formation by nuclear magnetic resonance is unanimously acknowledged to be a diagnostic aid. Of course, it can be used in other fields. The goal of the present invention is to help in the creation of images, of a body to be examined, which are more faithful and more precise in their resolution.
A device for optical image formation by nuclear magnetic resonance comprises essentially three types of coils. An initial type of coil (which may be replaced if necessary by a permanent magnet) is aimed at creating a strong, homogenous, magnetic field B.sub.0 in a pre-determined space of interest. A second type of coil, known as a radiofrequency coil, is aimed at subjecting a body, which is examined and placed under the influence of the field of the first coils, to radiofrequency excitation sequences and at measuring a radiofrequency signal transmitted in return by the particles of the body. The radiofrequency response is a response in volume: all the particles of a region of the body subjected to the examination transmit their radiofrequency responses at the same time. To create an image, it is necessary to differentiate these responses. To this end, optical image formation devices comprise a third type of coils, known as gradient coils, to superimpose the component of an additional magnetic field on the the strong field. The value of these components is a function of the coordinates in space of their place of application (conventionally, it is proposed to structure this differentiation along three mutually perpendicular axes, X, Y and Z; by convention, the axis Z is even generally taken as being co-linear with the strong field created by coils of the first type). In other words, each location of the space may be coded by a different field value; the modifications that result from this are exploited in the re-transmitted signal to create the image.
Gradient coils are generally divided into three classes: those that create a gradient along X, those that create a gradient along Y and those that create a gradient along Z. For example, a field gradient along X is a magnetic field for which the distribution of the co-linear component at the strong field (Z) in space is a function solely of the coordinate xi of its place of application. In practice, it is preferably even proportionate to this coordinate. This means that all the particles of a body to be examined, located in a plane parallel to Y-Z and with a given abscissa x.sub.i, are subjected to one and the same total field B.sub.0 +G.sub.x .multidot.x.sub.i. The gradient G.sub.x is the slope of the variation of the component along Z of the additional field provided by these X gradient coils.
The acquisition of an image, therefore, requires, during the application of radiofrequency excitation sequences, the concomitant application of field gradient sequences. The field gradient sequences depend on the method of optical image formation used. This method may, for example, be of the 2DFT type prepared by Mr. A. KUMAR and Mr. R. R. ERNST or, for example, of the back projection type prepared by Mr. P. C. LAUTERBUR. Regardless of the method of optical image formation chosen, one characteristic of field gradients is that they are pulsed. They are set up, they persist for a short period and then they are cut off. This may occur one or more times during each sequence. The consequence of this specific feature is that the functioning of the coils which produce them must be studied not only during permanent operation, during the application of the gradients, but also during the transients which result from their establishment and their cutting off. Another major characteristic of field gradients relates to their homogeneity. Homogeneity is taken to mean compliance with a given tolerance value and, through a real field gradient, compliance with an ideal theoretical distribution which is sought to h=imposed: the divergence of the field results in falsifying the differentiation that is sought to be imposed in space, this differentiation being the very basis of optical image formation. From this point of view, problems of homogeneity have to be resolved as much for the gradients as for the strong, homogeneous fields. The solution of the problems raised by transients is resolved, in principle, by making coils as small as possible. The smaller they are, the lower is their self-inductance and the smaller is the amount of power needed to produce the field gradient. In contrast, for homogeneity, the bigger the coils, the more can the distribution of the fields that they produce be considered to be homogeneous. These two tendencies are, therefore, contradictory. In general, the gradient coils are placed outside, as near as possible to the radiofrequency coils. These coils determine, in their interiors, the volume of examination in which the body is placed. The problem to be resolved then, for an imposed location, is to find gradient coils which produce a sufficiently steep (G.sub.x) homogeneous gradient and which have low self-inductance.